1. Field of the Invention
The present invention relates to a technique for designing a semiconductor circuit device, and particularly, to a technique for designing a semiconductor circuit device that enables more efficient design.
2. Description of the Related Art
With the progress of the communication technology and the semiconductor technology, mobile devices such as cellular phones and personal digital assistants (PDA) have spread widely. Typical mobile devices are driven by a rechargeable battery to comply with the portability. Some techniques for reducing power consumption of the mobile devices are developed to extend battery life and reduce charging frequency and time. One technique is to turn off or reduce a supply voltage to an unused circuit area of a chip so as to reduce unnecessary power consumption. To turn off or reduce a supply voltage to a specific circuit area, the circuit in the chip is divided into several circuit areas, each of which belongs to a different power system. Each power system may be controlled separately.
This kind of chip has a problem of shoot-through current between the circuit areas driven by different power supplies when one power supply is OFF. Some techniques to solve this problem have been developed, including the one disclosed in Japanese Unexamined Patent Application Publication 2001-148625, for example. According to this technique, a semiconductor integrated circuit has an input circuit operating by a first power supply and an output circuit operating by a second power supply with varying output voltages and having a CMOS inverter for supplying a High or Low level output signal to the input circuit. The output signal from the CMOS inverter of the output circuit is input to a control unit in the input circuit. If the second supply voltage is equal to or higher than a given voltage, the control unit outputs the output signal from the CMOS inverter as an internal signal of the input circuit. If the second supply voltage is less than the given voltage, on the other hand, the control unit outputs a Low level signal as an internal signal of the input circuit.
In this circuit structure, while the second power supply for the output circuit is OFF, the output of the control unit in the input circuit is kept Low, thereby preventing the shoot-through current from occurring in the input circuit. If the second power supply turns from ON to OFF when the output signal from the CMOS inverter is High, the output signal of the CMOS inverter turns from High to Low through an intermediate level. Since the output of the control unit turns to Low when the second supply voltage falls below the given voltage, no shoot-through current occurs in the input circuit. The power consumption is thereby reduced.
As described above, several circuit techniques to solve the problem in the different power supply circuit areas have been developed. However, the circuit design of the chip having a plurality of circuit areas in different power systems has not been addressed. Efficient and easy design is important particularly in the design of semiconductor circuit devices using a design system. Conventional design techniques, however, require a user to insert the circuit between the circuit areas after designing the different circuit areas. It is thus unable to perform efficient circuit design. Further, since the conventional design techniques insert the circuit between the circuit areas after separately designing the transmitting section and the receiving section of the different circuit areas, it is unable to control the line length, causing signal delay.